X-ray absorption spectroscopy is a widely used technique to determine local atomic structure and/or electronic structure of matter. X-ray absorption spectroscopy data are obtained by measuring transmission and/or characteristic fluorescent x-rays of an element in a material as a function of incident x-ray energy over an energy range with sufficiently narrow energy band that corresponds to an absorption edge of an element of interest, at which the incident x-ray photon has sufficient energy to excite core electron(s). The local atomic structure and electronic structure of the material show resonances and exhibit interference effects that affect the x-ray transmission as a function of the incident x-ray energy, so, by measuring the x-ray transmission as a function of x-ray energy, information about the local atomic structure and/or the electronic structure of the material can be determined.
Absorption spectra near x-ray absorption edges are generally divided into the “near edge” region, comprising the first 100 eV near an absorption edge, and the “extended” region, up to 1000 eV higher. Near edge spectroscopy, generally referred to as near edge x-ray absorption fine structure (NEXAFS) spectroscopy, or x-ray absorption near edge spectroscopy (XANES), are the result of transitions of core electrons to low lying unoccupied energy levels, and often has sharp resonance peaks. It is of use in understanding the chemical environment, and in particular the oxidation state of the element involved. Spectroscopy of the more extended energy region, generally referred to as extended x-ray absorption fine structure (EXAFS), exhibits gentle oscillations of absorptivity with energy. These arise from the interactions of the photoelectrons emitted with the surrounding atoms within the structure. From the analysis of EXAFS spectra, distances between atoms can be determined with high precision. The experimental analysis focusing on both spectral regions has been referred to as x-ray absorption fine structure (XAFS).
FIG. 1 illustrates a conventional synchrotron XAFS measurement apparatus, in which a synchrotron x-ray source P80 produces a beam of collimated x-rays P889 (typically collimated using an aperture or pinhole). The x-ray optical system associated with the source P80 may also include beam stops, apertures, or other optical elements to shape the x-ray beam P889 to have particular properties. The collimated x-ray beam P889 then enters a double crystal monochromator P330 comprising a first crystal P330A, which reflects the desired spectral component 889-1 onto a second crystal P330B to further limit the spectral bandwidth to produce a monochromatized x-ray beam 889-2, which is then incident on a sample 240 to be measured. The transmitted x-rays P889-X that pass through the sample 240 are then detected by a detector P290, which will typically transmit its signals to signal processing electronics P292 that then are further processed by an analysis system P295, which often has a display P298 to show data and results to the user. Motion controls for the double crystal monochromator P330 (not shown) are typically provided so that adjustment of the angle relative to the x-ray beam will select a particular narrow range of x-ray energies for transmission through the monochromator P330.
FIG. 2 shows an illustration of XAFS results, in this case for a model iron oxide (Fe2O3) powder. Data are collected for x-rays with energy near the iron (Fe) K-edge absorption at 7.112 keV. Near edge structure (XANES) is clearly visible, as are the higher energy EXAFS variations from the standard absorption (denoted by μ(E)). From these variations, details of the chemical state and inter-atomic structure of the material may be derived. [Data in FIG. 2 adapted from the PhD dissertation of Mauro Rovezzi, “Study of the local order around magnetic impurities in semiconductors for spintronics.” Condensed Matter, Université Joseph-Fourier—Grenoble I, 2009. English. <tel-00442852>.]
Typically, an x-ray beam with an energy bandwidth of less than 1 eV is required for XANES and less than 10 eV for EXAFS measurements. In electron bombardment x-ray sources, the x-ray spectrum typically consists of a continuous spectrum (known as Bremsstrahlung radiation) along with sharp characteristic x-ray fluorescence lines of element(s) in the anode illuminated by the incident electron beam. For XFAS measurement, Bremsstrahlung radiation is typically used. Because intensity oscillations in EXAFS spectra are often less than 10% of the total absorption, and structural parameters can be obtained only after a Fourier transform analysis of the data, highly accurate (0.1% or better) absorption measurements are often required. Therefore, at least a million photon counts are typically required at each energy datapoint.
To meet the narrow energy bandwidth required, a crystal monochromator can be used. Several methods can be used to obtain the required energy bandwidth. One method is to use a pair of flat crystals with appropriate reflection Miller index, such as silicon Si (111), arranged to diffract an incident x-ray beam with a small convergence angle in the dispersion plane (containing the incident and diffraction beam). The required small angular convergence can be obtained for example using a pair of narrow slits separated by a predetermined distance. Sometimes, only one slit is required if the source size in the dispersion plane is small. This method offers several advantages: simple energy scanning by scanning the x-ray beam incidence angle by rotating the monochromator, and a fairly wide energy scanning range using a single crystal, often without need to move the source and the sample in positions and angles, which can be important for many experiments, and results in substantially simpler system and lower cost.
The small convergence angle means that this technique can be widely used for experiments using synchrotron radiation sources, which have high brightness and are highly directional. However, in a laboratory setting, where x-rays are generated in an electron bombardment source, only a small fraction of the x-rays will be collected and used. Consequently, the x-ray flux of a monochromatic x-ray beam obtained using this method from a conventional electron bombardment x-ray source has simply been too low to allow laboratory XAFS measurement.
To circumvent the low x-ray flux and thus low throughput problem, most laboratory source based XAFS systems employ a single bent crystal monochromator in a Rowland circle geometry. In such a system, the x-ray source, the bent crystal, and the slit in front of the sample, are all located on the Rowland circle, with appropriate orientation. Although using a bent crystal can increase the angular acceptance of x-rays generated by the source, Rowland circle based x-ray spectrometers suffer the following drawbacks: difficulty to obtain high x-ray energy resolution due to crystal strain induced by the bending, the requirement of moving at least two of the three major components (source, crystal and slit/sample) for energy scanning. In addition, a single bent crystal covers only a relatively small energy range, so in practice often a set of bent crystals are required. The use of Rowland circle based spectrometers may in part be motivated by the lack of x-ray sources of small size with sufficient brightness, and the lack of x-ray optics for collecting x-rays generated in the source with a large solid angle able to provide an x-ray beam with sufficient angular collimation.
Another performance limiting factor of Rowland circle based monochromator is high order harmonic contamination of the monochromatic x-ray beam because the bent crystal often reflects x-rays of energy equal to an integer multiple of the desired x-ray energy. To circumvent this higher order harmonic contamination problem, low energy electron beam excitation is often used. The use of low energy electrons directly leads to a decrease of x-ray flux because x-ray production efficiency of bremsstrahlung radiation in an electron bombardment x-ray source is approximately proportional to the incident electron energy.
The low performance of conventional laboratory-based x-ray absorption spectroscopy systems has severally limited their utilization. X-ray absorption spectroscopy measurements are currently carried out mostly at synchrotron radiation facilities, which offer high throughput because of their large source brightness and usable x-ray flux over a wide energy range. However, synchrotron radiation sources are large and expensive, often occupying acres of land, and only available in a limited number of locations. As a consequence, access per user group is typically finite and infrequent, particularly for industrial users concerned about protecting intellectual property and/or difficulty justifying important but routine measurements in competition with academic research proposals for lack of pure scientific merit. Other limitations include difficulty in transportation to synchrotron radiation facilities of delicate samples (e.g., fragile or sensitive to oxidization), severe biological and radiological safety considerations, or in-situ experiments requiring special instrumentation that may be unavailable.
XAFS techniques have many applications. In particular, EXAFS can measure local interatomic distances in crystalline and non-crystalline materials. This is a unique capability for non-crystalline materials, such as amorphous solids, solid solutions, dopant and implant materials in semiconductor devices, catalysts, liquids, and organometallic compounds. XANES is particularly useful in determining the oxidation states of the element in its chemical environment. However, these techniques suffer from the lack of a high brightness compact x-ray source that can be used in a laboratory or in the field. There is therefore a need for such a high brightness, compact x-ray system for XAFS measurements.